I’ve read that there may be as much as 10 times more water in the earth than on it. In fact the story I read seemed to suggest that our sea levels are as high as they can be because more water in the oceans would mean more water forced into the earth.
What I’ve noticed is that those who claim the Earth is gaining or losing water (seas rising or falling) cannot explain where this water would come from or go to.
Between the minor amount of water evaporated into space and the ice crystals which we attract as we spin through our solar system, I don’t think the amount of water on Earth has changed significantly in a long, long time.